Hong Kong
Hong Kong Ovulation calculator
There are only a handful of days in each cycle when pregnancy is possible, and this tool pinpoints them. Give it the first day of your last period, your usual cycle length and, if you know it, your luteal phase, and it works out the day you are most likely to ovulate plus the fertile window around it. That window matters because sperm can survive several days inside the body while the released egg lasts under a day, so the best chances come from the days leading up to ovulation rather than the day itself. People use it to time trying for a baby, to understand their own cycle, or to know when a period is next due.
How it works
- Enter the first day of your most recent period.
- Set your average cycle length, measured from the first day of one period to the first day of the next.
- Leave the luteal phase at 14 days unless you know yours differs; it is the gap between ovulation and the next period.
- Ovulation day is placed one luteal phase before that next period, so cycle length minus luteal length gives the day of the cycle.
- The fertile window is shown as the five days before ovulation through to the day after, with the next period date alongside.
ovulation day = cycle length - luteal phase; fertile window = day - 5 to day + 1
The luteal phase, from ovulation to the next period, holds fairly constant at about 14 days, while the first half of the cycle stretches or shrinks with cycle length. Counting one luteal phase back from the expected next period therefore lands on ovulation. The fertile window opens five days earlier, the longest sperm reliably survive, and closes a day after release, covering the egg's short life.
- cycle
- days from the start of one period to the next
- luteal
- days from ovulation to the next period, about 14
- -5 to +1
- the fertile window around ovulation day
Cycle and fertility reference points
| Typical cycle length | 21 to 35 days | anything in this range is considered normal |
| Fertile window | ≈ 6 days | five days before ovulation plus the day itself |
| Egg lifespan | 12 to 24 hours | after it is released |
| Sperm lifespan | up to 5 days | in fertile cervical mucus |
Worked example
A period starting 1 March 2026, a 28-day cycle and a 14-day luteal phase: ovulation lands on cycle day 14, which is 14 March, and the fertile window runs from 9 March to 15 March. The next period would be expected around 29 March. On a 30-day cycle, ovulation slips to day 16 and the window moves with it.
Key facts
- Ovulation is timed backwards from the next period, not forwards from the last one, because the luteal phase is the steadier half.
- The days before ovulation matter most, since sperm can already be waiting when the egg arrives.
- A cycle that is longer than 28 days pushes ovulation later, not the period earlier.
- Stress, illness and travel can delay ovulation in any given month, which is why dates alone are only a guide.
Tips
- Track the first day of several periods to get a true average cycle length before relying on the prediction.
- For the best chance of conceiving, focus on the two or three days before the predicted ovulation date.
- Pair the estimate with ovulation predictor tests if you want a sharper signal in the moment.
- If your cycles run shorter or longer than 21 to 35 days, or vary wildly, mention it to a clinician.
Frequently asked questions
When am I most fertile?+
The two or three days right before ovulation and the day of ovulation give the highest chance, because sperm are already in place when the egg is released. The window in the result reflects this rather than a single day.
How long does an egg survive?+
About 12 to 24 hours after release. Sperm, by contrast, can live for up to five days in fertile cervical mucus, which is why the days before ovulation count more than the days after.
My cycle is irregular, can I still use this?+
You can, but the estimate loses accuracy when cycle length jumps around. Ovulation tests that detect the hormone surge, or tracking your waking temperature, give a clearer signal than dates alone.
Can I rely on this to avoid pregnancy?+
No. Calendar timing is not a reliable form of contraception on its own, since ovulation can shift and sperm survive for days. Speak to a clinician about methods proven to prevent pregnancy.
What is a luteal phase?+
It is the stretch from ovulation to the start of your next period, usually around 14 days and fairly steady for an individual. Most of the cycle-to-cycle variation happens before ovulation, not after it.
Things to watch
- This is general information, not medical advice, and not a contraceptive method.
- Ovulation can move within a cycle, so the window is a best estimate rather than a guarantee.
- If you have been trying to conceive for a year, or six months over the age of 35, seek advice from a doctor.
Sources
- Trying to get pregnant: when am I most fertile? · NHS
- Timing of sexual intercourse in relation to ovulation · American College of Obstetricians and Gynecologists
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with the official sources listed before making decisions.
Reviewed by Vikas Dulgunde.